[llvm] [DAGCombiner] Add generic DAG combine for ISD::PARTIAL_REDUCE_MLA (PR #127083)

[Benjamin Maxwell via llvm-commits]

https://github.com/MacDue approved this pull request.

LGTM, thanks :+1: I think the Neon regressions should be fixed after the lowering changes. 

https://github.com/llvm/llvm-project/pull/127083